Marc, Michael, Yuhang

In the past, we usually check the mode matching between filter cavity (FC) reflection and homodyne LO without considering the beam jittering. However, FC reflected beam jittering is an issue which degrades homodyne detection efficiency.

To check this issue, we first lock filter cavity with green with AA/pointing/length control loops on. Then we half-detune BAB and check its spectrum on oscilloscope when it arrives AMC. Due to jittering, there are peaks going up and down in the AMC spectrum. We used oscilloscope persisit function to record the spectrum for about 200 seconds (as attached figure). In this situation, we measure the highest value of these peaks.

The peaks height are

All the peaks in the above table are taken in the same manner. But we firstly took TEM00, then we zoomed in and checked higher order modes. Since they are taken in the same manner, we do division as (all HOMs)/(all HOMs+TEM00) = 10.57%.

If we use this value, the mode mismatch in homodyne detection will be 0.8943*0.8943 = 80%. Considering the optical loss in elog2511, the total optical losses will be 1-0.8*0.807*0.904*0.99 = 42.2%. This value is larger than the optical losses we used in PRL paper, but closer to the derived optical losses from SQZ/ASQZ measurement in this link. However, it is noted that the evaluation of mode mismatch in this entry should be a pessimistic one. Because we take the highest HOMs, which only tells us the worst mode matching in the 200s measurement. We also conceived to take many instant AMC spectrum of FC reflected BAB, which should give a more reasonable evaluation.

In addition, we can also use visibility measurement to double check the mode mismatch induced homodyne in-efficiency.